Comment by tianon

I have my router serving up TFTP with the following pxelinux.cfg (and the appropriate vmlinuz64 and initrd.img extracted from the b2d ISO):

  LABEL boot2docker
          MENU LABEL boot2docker v0.5.2
          KERNEL boot2docker/v0.5.2/vmlinuz64
          APPEND initrd=boot2docker/v0.5.2/initrd.img loglevel=3 user=docker